The present invention relates to a circuit for the filtering of a pulse signal. It can be applied especially in chip cards or smart cards, notably for the filtering of the clock signal.
The invention relates especially to circuits that require a pulse signal for which the high and low states of the pulses are stable for a minimal fixed period. This is especially the case with circuits designed for chip cards such as credit cards or phone cards for which it is absolutely necessary to foresee and prevent any fraud. Now, without the efficient and reliable filtering of the clock signal applied to a chip card, a fraudulent individual could interfere with this external clock signal by means of very short pulses which, if taken into account internally by the circuit, could for example enable the fraudulent individual to gain access to unauthorized functions.
A known way of carrying out filtering operations such as these is to use phase-locking circuits with low-frequency filters. However, these circuits are complex and they most usually require discrete components, notably for making low-frequency filters. Their operation is reliable for characteristics of the input signal that are well delimited within a restricted and specified zone. Beyond this zone, their reliability is no longer assured.
To resolve these technical problems, the invention proposes a circuit for filtering a pulse signal that delivers a pulse signal at output for which the high and low states of the pulses have a guarantied minimal duration that is independent of the characteristics of the input signal.
According to the invention, the generation of an output pulse is activated by the detection of an input pulse and the form of the output pulse is based on elementary delays obtained by the charging of capacitors. However, throughout the duration of the elementary delays, no input pulse can be taken into account.